Method for manufacturing high-strength spring

ABSTRACT

The present invention intends to provide a method for manufacturing a high-strength spring, which is capable of generating a higher level of compressive residual stress than that given by conventional methods. This object is achieved as follows: After the final heating process, such as the tempering (in the case of a heat-treated spring) or removing-strain annealing (in the case of a cold-formed spring), a shot peening process is performed on the spring while the surface temperature of the spring is within the range from 265 to 340° C. (preferably from 300 to 340° C.). Subsequently, the spring is rapidly cooled. Preferably, a prestressing process is performed before the shot peening process, or after the shot peening process and before the rapid cooling process. The rapid cooling process may be either a water-cooling process or an oil-cooling process. A forced-air cooling process may be used if the wire diameter of the spring is small.

TECHNICAL FIELD

The present invention relates to a shot peening method for manufacturinga spring, particularly a suspension spring, having a high level ofdurability (or fatigue resistance) and sag resistance.

BACKGROUND ART

As a method for remarkably improving the durability of a spring, shotpeening is an indispensable process for a high-strength spring,especially for a suspension spring used in automobiles or a valve springused in engines.

In the shot peening process, a number of small particles are projectedonto the surface of the target object. This process is apparently thesame as the shot blast, a process that is performed to make the surfaceclean by removing burrs (or projections) resulting from cutting orforming work or scales (i.e. a hard oxide layer) resulting from a heattreatment. However, the two processes significantly differ from eachother in respect to the strength and other conditions; for shot peening,the conditions are determined to cause a plastic deformation only on thesurface of the spring so that a compressive stress remains on thesurface.

The main purpose of shot-peening a spring is to generate beforehand acompressive residual stress within the surface of the spring so that theload stress working on the spring when it is in service is reduced by anamount equal to the residual stress. For this purpose, various shotpeening methods have been developed to attain as high a residual stressas possible.

For example, the Japanese Examined Patent Publication No. S48-20969discloses a technique in which a piece of spring steel having a sorbitestructure is shot-peened under a warm environment with a temperature of200 to 400° C. after the quenching and tempering processes.

The Japanese Unexamined Patent Publication No. S58-213825 discloses atechnique in which the shot peening is performed while the temperatureof the spring is within the range from 150 to 350° C. in the course ofthe cooling process after the temper-heating process.

The Japanese Unexamined Patent Publication No. H05-140643 discloses atechnique for generating an adequate level of compressive residualstress, in which a piece of steel having a predetermined compositionundergoes a warm shot peening process while the temperature ismaintained within the range from 150 to 300° C. after the thermalrefining process, i.e. the quenching and tempering processes.

The techniques disclosed in the aforementioned three publications werefirst developed in the days when springs were used under low levels ofworking stress. Such past techniques could not always meet theperformance requirements for the latest springs that were put in serviceunder much higher levels of working stress.

To solve such a problem, the present invention intends to provide amethod for manufacturing a high-strength spring, which is capable ofgenerating a higher level of compressive residual stress than thatgenerated by conventional methods.

DISCLOSURE OF THE INVENTION

To solve the above-described problem, the method for manufacturing ahigh-strength spring according to the present invention is characterizedby:

a shot peening process performed on the spring while the surfacetemperature of the spring is within the range from 265 to 340° C., and

a rapid cooling process performed on the spring after the shot peeningprocess.

It is preferable to perform a setting process before the shot peeningprocess, or after the shot peening process and before the rapid coolingprocess.

The rapid cooling process may be either a water-cooling process or anoil-cooling process. A forced-air cooling process is also available ifthe wire diameter of the spring is small.

The above-described method exhibits a more remarkable effect if it isapplied to a spring made of a steel material containing, in weightpercentage, 0.35 to 0.55% of C, 1.60 to 3.00% of Si, 0.20 to 1.50% ofMn, 0.010% or less of S, 0.40 to 3.00% of Ni, 0.10 to 1.50% of Cr, 0.010to 0.025% of N and 0.05 to 0.50% of V, with Fe substantiallyconstituting the remaining percentage.

To improve the energy efficiency, it is preferable to perform theabove-described process when the spring is cooled after a certain kindof heating process is performed on the spring. For a spring that needs aheating treatment (i.e. quenching and tempering), the aforementioned“heating process” means the final heating process (i.e. the tempering).For a spring that does not need such a heating treatment, the “heatingprocess” means some other kind of heating process, an example of whichis a removing-strain annealing performed after a cold-working process(e.g. coiling process). For a warm-formed spring, the temper heating isusually performed at a temperature within the range from 400 to 450° C.For a cold-formed spring, the removing-strain annealing that follows thecoiling process is performed at a temperature within the range from 350to 450° C. Therefore, the shot peening, prestressing and other necessaryprocesses can be performed within the temperature range specifiedearlier. It is allowable to provide an additional heating step apartfrom the “heating process.” In this case, the shot peening and relatedprocesses may be performed while the heating operation is maintained,not in the course of a cooling process after the heating operation isstopped.

If the shot peening is performed in a warm environment where the springstill has a high temperature, the hardness of the spring (or work piece)relative to that of the shot particles becomes lower than that observedin the case where the shot peening is performed in a cold environment.Therefore, the shot peening produces a greater magnitude of plasticdeformation on the surface of the spring, thereby generating a highlevel of compressive residual stress within the surface. It also makesthe compressive residual stress to develop more deeply from the surface.

In conventional methods, the spring is made to cool naturally after thewarm shot peening. For example, if, as in the case of a suspensionspring, the wire diameter of the spring is as large as 10 to 15 mm, ittakes more than five minutes for the temperature to fall from 300 to200° C. Leaving the spring under such a warm environment for such a longtime will cause a relaxation of the high compressive residual stress.

In the method according to the present invention, a rapid coolingprocess immediately follows the shot peening process performed at theabove-specified temperature range. Therefore, the high compressiveresidual stress resulting from the warm shot peening is maintained untilthe spring reaches the room temperature. Thus, the spring manufacturedby the method according to the present invention gains a higher level ofdurability.

The previous discussion also applies to the prestressing process. Oneobject of performing the prestressing in a warm environment is to causebeforehand, in the course of the production, a plastic deformation (orsag) that can occur in the future while the spring is in service, and toimmobilize beforehand any dislocations that may cause a plasticdeformation. Performing a slow cooling process after the warmprestressing process allows the dislocations to move again while thetemperature is high, which will cause the spring to sag in the future.In contrast, in the method according to the present invention, the rapidcooling process that immediately follows the warm prestressing processassuredly immobilizes the dislocations, so that only a minimal amount ofsag is allowed to occur later while the spring is in service.

Furthermore, compared to the cold prestressing performed after thespring is cooled, the warm prestressing reduces the amount ofcompression of the spring necessary to create the same magnitude ofpermanent deformation. This effectively improves the evenness in theform (e.g. the free length and the bowing) of the spring observed afterthe prestressing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a table showing the chemical composition of a sample spring.

FIG. 2 is a flowchart showing the process of manufacturing the samplespring.

FIG. 3 is a table showing the dimensions of the sample spring.

FIG. 4A is a graph showing the relationship between the temperature atthe exit of the temper furnace and the temperature of the work piece,and FIG. 4B is a graph showing the relationship between the temperatureat the exit of the temper furnace and the free length of the work pieceobserved after a warm prestressing process.

FIG. 5 is a graph showing the compressive residual stress distributionon the surface of rapidly cooled samples.

FIG. 6 is a graph showing the compressive residual stress distributionon the surface of naturally cooled samples.

FIG. 7 is a graph showing the result of a corrosion resistance test ofthe sample spring.

BEST MODE FOR CARRYING OUT THE INVENTION

A test for confirming the effect of the method according to the presentinvention was conducted using a steel material having a chemicalcomposition shown in FIG. 1. Several pieces of coil springs weremanufactured by a process shown in FIG. 2. The dimensions of the coilsprings are shown in FIG. 3.

As shown in FIG. 2, the test samples were divided into two groups (A)and (B). The sample springs belonging to group (A) were prestressed andshot-peened in a warm environment where the temperature of the springswas within the range from 265 to 340° C. Then, the springs weresubmerged under water for rapid cooling. In contrast, the springs ofgroup (B) were naturally cooled (or air-cooled) after being prestressedand shot-peened in the same manner. The shot peening was performed underthe following condition: arc height=0.37 mm, coverage=100%.

A tempering treatment for a spring includes the step of maintaining aquenched spring at a predetermined tempering temperature for a specifiedperiod of time. In general, the process of manufacturing springs formass-production uses a conveyor-type temper furnace. This type offurnace allows the temperature at its exit to be set at desired valuesafter the tempering process is performed at a predetermined temperaturefor a predetermined period of time. This means that the temperature ofthe spring (or work piece) can be set as desired for the warm shotpeening process and the warm prestressing process. Therefore, researchwas conducted on the relationship between the temperature at the exit ofthe temper furnace and the temperature of the spring (or work piece)observed immediately after they had exited the furnace. The result isshown in FIG. 4A, which demonstrates that a rise in the temperature atthe exit of the furnace improves the evenness in the temperature of thework.

FIG. 4B shows the relationship between the temperature at the exit ofthe same furnace and the free length of the spring observed after thewarm prestressing process. It also demonstrates that a rise in thetemperature at the exit of the furnace improves the evenness in the freelength of the work piece. This is because the warm prestressing reducesthe amount of compression of the spring and accordingly lowers the levelof stress applied to the spring.

The above-described results demonstrate that it is possible tomanufacture springs having an improved evenness in form by setting thetemperature at the exit of the temper furnace high enough for thetemperature of the spring to be as high as 265 to 340° C. (preferably300° C. or higher) during the warm prestressing process and the warmshot peening process.

Next, the characteristics of the springs manufactured as described abovewere examined. For the water-cooled group (A), three kinds of springswere manufactured by setting the temperature at the beginning of theshot peening process to three different values: 265, 305 and 340° C.FIG. 5 shows the result of measuring the residual stress distributionfrom the surface to a depth of 0.5 mm for each of the three kinds ofsprings. Every spring exhibits the maximum compressive residual stressof over 1000 MPa. Moreover, the stress does not fall below 800 MPa untilthe depth reaches a level of 0.3 mm.

For the naturally cooled group (B), three kinds of springs weremanufactured by setting the temperature at the beginning of the shotpeening process to three different values: 265, 305 and 340° C. FIG. 6shows the result of measuring the residual stress distribution from thesurface to a depth of 0.5 mm for each of the three kinds of springs.Again, every spring exhibits the maximum compressive residual stress ofover 1000 MPa. However, except for the spring treated under thetemperature of 265° C., the stress falls below 800 MPa when the depthreaches a level of about 0.15 to 0.20 mm.

It is possible to carry out the shot peening process a plurality oftimes. A shot peening process may be a stress peening process, whenevernecessity.

FIG. 7 shows the result of a corrosion resistance test performed on thesprings of the two groups (A) and (B). The test was conducted under theconditions specified in the figure. FIG. 7 clearly shows that thesprings rapidly cooled after the warm shot peening and warm prestressingprocesses have higher levels of durability than those of the naturallycooled springs.

1. A method for manufacturing a high-strength spring, comprising: aheating process performed on the spring for heating the spring at atemperature within a range from 350 to 450° C.; a warm prestressingprocess performed on the spring after the heating process while asurface temperature of the spring is within a range from 265 to 340° C.;a shot peening process performed on the spring after the warmprestressing process while a surface temperature of the spring is withina range from 265 to 340° C.; a rapid cooling process performed on thespring after the shot peening process; and a cold prestressing processperformed after the rapid cooling process.
 2. A method for manufacturinga high-strength spring, comprising: a heating process performed on thespring for heating the spring at a temperature within a range from 350to 450° C.; a warm prestressing process performed on the spring afterthe heating process while a surface temperature of the spring is withina range from 300 to 340° C.; a shot peening process performed on thespring after the warm prestressing process while a surface temperatureof the spring is within a range from 300 to 340° C.; a rapid coolingprocess performed on the spring after the shot peening process; and acold prestressing process performed after the rapid cooling process. 3.A method for manufacturing a high-strength spring, comprising: a heatingprocess performed on the spring for heating the spring at a temperaturewithin a range from 350 to 450° C.; a warm prestressing processperformed on the spring after the heating process while a surfacetemperature of the spring is within a range from 265 to 340° C. whilethe spring is cooled after the heating process; a shot peening processperformed on the spring after the warm prestressing process while asurface temperature of the spring is within the range from 265 to 340°C.; a rapid cooling process performed on the spring after the shotpeening process; and a cold prestressing process performed after therapid cooling process.
 4. A method for manufacturing a high-strengthspring, comprising: a heating process performed on the spring forheating the spring at a temperature within a range from 350 to 450° C.;a warm prestressing process performed on the spring after the heatingprocess while a surface temperature of the spring is within a range from300 to 340° C. while the spring is cooled after the heating process; ashot peening process performed on the spring after the warm prestressingprocess while a surface temperature of the spring is within a range from300 to 340° C.; a rapid cooling process performed on the spring afterthe shot peening process; and a cold prestressing process performedafter the rapid cooling process.
 5. The method for manufacturing ahigh-strength spring according to claim 1, wherein the shot peeningprocess is performed a plurality of times.
 6. The method formanufacturing a high-strength spring according to claim 1, wherein astress peening process is performed in the shot peening process.
 7. Themethod for manufacturing a high-strength spring according to claim 1,wherein the rapid cooling process is a water-cooling process.
 8. Themethod for manufacturing a high-strength spring according to claim 1,wherein the aforementioned processes are performed on a spring made of asteel material containing, in weight percentage, 0.35 to 0.55% of C,1.60 to 3.00% of Si, 0.20 to 1.50% of Mn, 0.010% or less of 5, 0.40 to3.00% of Ni, 0.10 to 1.50% of Cr and 0.05 to 0.50% of V, with Fesubstantially constituting the remaining percentage.
 9. The method formanufacturing a high-strength spring according to claim 3, wherein theheating process is a temper-heating process performed in a quenching andtempering treatment.
 10. The method for manufacturing a high-strengthspring according to claim 3, wherein the heating process is a heatingprocess for removing-strain annealing performed after a cold-workingprocess.
 11. A high-strength spring, manufactured by a methodcomprising: a heating process performed on the spring for heating thespring at a temperature within a range from 350 to 450° C.; a warmprestressing process performed on the spring after the heating processwhile a surface temperature of the spring is within a range from 300 to340° C.; a shot peening process performed on the spring after the warmprestressing process while a surface temperature of the spring is withina range from 300 to 340° C.; a rapid cooling process performed on thespring after the shot peening process; and a cold prestressing processperformed after the rapid cooling process, wherein the spring is made ofa steel material containing, in weight percentage, 0.35 to 0.55% of C,1.60 to 3.00% of Si, 0.20 to 1.50% of Mn, 0.010% or less of S, 0.40 to3.00% of Ni, 0.10 to 1.50% of Cr and 0.05 to 0.50% of V, with Fesubstantially constituting the remaining percentage, and a duration ofthe spring in a corrosion fatigue test exceeds 60,000 cycles under astress of 659±438 MPa.
 12. A high-strength spring, manufactured by amethod comprising: a heating process performed on the spring for heatingthe spring at a temperature within a range from 350 to 450° C.; a warmprestressing process performed on the spring after the heating processwhile a surface temperature of the spring is within a range from 265 to340° C. while the spring is cooled after the heating process; a shotpeening process performed on the spring after the warm prestressingprocess while a surface temperature of the spring is within the rangefrom 265 to 340° C.; a rapid cooling process performed on the springafter the shot peening process; and a cold prestressing processperformed after the rapid cooling process, wherein the spring is made ofa steel material containing, in weight percentage, 0.35 to 0.55% of C,1.60 to 3.00% of Si, 0.20 to 1.50% of Mn, 0.010% or less of S, 0.40 to3.00% of Ni, 0.10 to 1.50% of Cr and 0.05 to 0.50% of V, with Fesubstantially constituting the remaining percentage, and a duration ofthe spring in a corrosion fatigue test exceeds 60,000 cycles under astress of 659±438 MPa.
 13. A high-strength spring, manufactured by amethod comprising: a heating process performed on the spring for heatingthe spring at a temperature within a range from 350 to 450° C.; a warmprestressing process performed on the spring after the heating processwhile a surface temperature of the spring is within a range from 300 to340° C. while the spring is cooled after the heating process; a shotpeening process performed on the spring while a surface temperature ofthe spring after the warm prestressing process is within the range from300 to 340° C.; a rapid cooling process performed on the spring afterthe shot peening process; and a cold prestressing process performedafter the rapid cooling process, wherein the spring is made of a steelmaterial containing, in weight percentage, 0.35 to 0.55% of C, 1.60 to3.00% of Si, 0.20 to 1.50% of Mn, 0.010% or less of 5, 0.40 to 3.00% ofNi, 0.10 to 1.50% of Cr and 0.05 to 0.50% of V, with Fe substantiallyconstituting the remaining percentage, and a duration of the spring in acorrosion fatigue test exceeds 60,000 cycles under a stress of 659±438MPa.
 14. A high strength spring, manufactured by a method comprising: aheating process performed on the spring for heating the spring at atemperature within a range from 350 to 450° C.; a warm prestressingprocess performed on the spring after the heating process while asurface temperature of the spring is within a range from 265 to 340° C.;a shot peening process performed on the spring after the warmprestressing process while a surface temperature of the spring is withina range from 265 to 340° C., and a rapid cooling process performed onthe spring after the shot peening process; and a cold prestressingprocess performed after the rapid cooling process, wherein the spring ismade of a steel material containing, in weight percentage, 0.35 to 0.55%of C, 1.60 to 3.00% of Si, 0.20 to 1.50% of Mn, 0.010% or less of S,0.40 to 3.00% of Ni, 0.10 to 1.50% of Cr and 0.05 to 0.50% of V, with Fesubstantially constituting the remaining percentage, a duration of thespring in a corrosion fatigue test exceeds 60,000 cycles under a stressof 659±438 MPa.
 15. The high strength spring according to claim 14,wherein the shot peening process is performed a plurality of times. 16.The high strength spring according to claim 14, wherein a stress peeningprocess is performed in the shot peening process.
 17. The high strengthspring according to claim 14, wherein the rapid cooling process is awater-cooling process.
 18. The high strength spring according to claim12, wherein the heating process is a temper-heating process performed ina quenching and tempering treatment.
 19. The high strength springaccording to claim 12, wherein the heating process is a heating processfor removing-strain annealing performed after a cold-working process.